Past research suggests that the parasite, estimated to infect 25 percent of people worldwide, can trigger or exacerbate psychotic symptoms and schizophrenia in genetically predisposed people.

The findings of the new study, published in the March issue of the journal Infection and Immunity, help explain why the infection causes serious disease in some but not in others and clarify its role in psychiatric disorders, the researchers say.

"We already know that toxoplasmosis can play a role in some psychiatric disorders, but up until now we didn't know why. Working with human nerve cells, our study shows the exact alterations triggered by each strain that can eventually manifest themselves as symptoms," says senior investigator Robert Yolken, M.D., a neurovirologist at the Johns Hopkins Children's Center.

The researchers injected human nerve cells with the three most common toxoplasma strains, each of which caused a different pattern of gene expression. One of the most basic functions in all living organisms, gene expression occurs when a gene is switched on to release a substance that tells cells what to do or not do, determining the cells' biologic behavior. Gene expression can be turned on and off, stepped up or down by various factors, including viral and bacterial invasions.

Cells infected with toxoplasma type I had the greatest impact on gene expression, altering more than 1,000 genes, 28 of them linked to brain development and central nervous system function and 31 others to nerve impulse and signaling.

Cells injected with the less virulent types II and III had low and moderate levels of gene expression. Infection with toxoplasma type II affected 78 genes, some of which were related to growth, certain hormones and circadian rhythm, while type III altered 344 genes, some of which were linked to metabolism.

A handful of genes were affected by all three strains, notably a gene called VIPR2 that regulates neurotransmitters and nerve signaling and may play a role in schizophrenia, the researchers say.

If these findings are confirmed in clinical studies with people, they could help physicians predict the severity of an infection by strain type and tailor treatments accordingly, the researchers say.